Cell Membranes
Structure, Chemistry and Characteristics
1
Introduction
• This presentation consists of 18 slides
• Use it a an outline for review of what you need to know
about cell membranes, including the plasma membrane, the
outer boundary of the cell.
• Study artist rendition of the plasma membrane in the next
slide and try to identify: cholesterol, the phospholipids, the
glycolipids, the proteins and the glycoproteins. Notice that
all the oligosaccharides are in the layer facing the
extracellular fluid.
• For additional help consult a college biology text, college
A&P text or a biochemistry text.
2
The Plasma Membrane
(Guyton Ch. 2.3 or Marieb ch. 2)
3
Significance of Carbohydrate Coat on
Cell Surface (Alberts Fig. 11.39)
• Most plasma membrane
proteins have bound to
them oligosaccharide
residues that face the
extracellular fluid.
• These sugars play a key
role in cell to cell
recognition and function
as markers for some cells.
• Example: Blood types A,
B and AB
4
Different Views of Cell Membranes
Notice Thickness in nanometer!
5
Functions of Membrane Proteins
This summarizes some of the major
functions carried out by
membrane proteins.
Transporters move solutes
across the membrane, or form
passageways/channels for ions.
Anchors attach cells to each
other, to the extracellular matrix
and to the cytoskeleton.
Receptors bind specific ligands
and enzymes like adenylyl
cyclase and phospholipase A
catalyze the synthesis of second
messengers like cAMP, inositol
triphosphate (IP3) and
diacylglycerol (DAG)
6
Plasma membrane: The Lipid Bilayer (I)
• As illustrated in the previous slides, the lipid
bilayer is composed of a variety of phospholipids,
glycolipids, cholesterol (in animal cells but other
sterols in plant cells and in bacteria). The type of
phospholipid and the relative abundance of a
phospholipid varies with the membrane within the
cell and also among different cells. (Will illustrate
in lecture).
• The next slide lists some of the major
phospholipids found in cell membranes
7
Plasma Membrane Lipid Bilayer (II)
Composition
• Some of the major phospholipids in the plasma membrane
include:
–
–
–
–
–
–
Phosphatidylcholine (24%)
Phosphatidylserine (4%)
Phosphatidylethanolamine (17%)
Phosphatidylglycerol
Sphingomyelin (19%)
Phosphatdylinositol (Significance in cell-cell communication
• Cardiolipin (diphosphatidylglycerol is found in inner mitochondrial
membrane and in chloroplasts, but NOT in the plasma membrane)
• Glycolipid (like the gangliosides and cerebrosides)
• Cholesterol (in animal cells but phytosterols and hopanoids
in bacteria)
– As illustrated in the next slide, all of these lipids are amphipathic
8
Membrane Lipids Are Amphipathic
9
Amphipathic Phospholipids form a
Bilayer in Water (Fig. 11.11)
10
Arrangement of Chlesterol in Lipid
Bilayer: Significance (Fig. 11.16)
11
Phospholipids Aggregate to Form Cell
Membranes (Fig. 2.20)
12
Phospholipid Bilayers Form Sealed
Compartments (Fig. 11.12)
13
Lipososmes: Significance in Medicine
(Fig. 11.13)
14
Significance & Characteristics of the Lipid
Bilayer
• The lipid bilayer forms a permeability barrier for solutes
that are relatively large, hydrophilic and charged
• The lipid bilayer is:
1.
2.
3.
Fluid (because its components are NOT static (see # 16)
Assymetrical with respect to composition of each monolayer (see
#17)
Impermeable to solutes that are polar, charged and relatively
large
4. Selectively permeable implies that the lipid will select
what solutes diffuses through the membrane on the
basis of 3 solute criteria: a) partition coefficient, b)
MW, c) charge and/or polarity.
15
Types of Movements Within the Plane of
the Membrane (Fig. 11.15)
16
Membrane Assymetry
(Alberts et al, fig. 11.17)
• Assymetry refers to the unequal
distribution of the lipids
between the two monolayers as
illustrated by the different
colors (different lipids) in the
monolayer facing the cytosol vs
that facing the extracellular
space.
• Thin Layer Chromatography
(TLC) is a t echnique used to
isolate & purify membrane
lipids: Ask Dr. Faitar for some
detail and/or illustration
17
THE END
Cell Membranes
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